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<li class="toctree-l1 current"><a class="current reference internal" href="#">Notes</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#why-can-t-i-just-use-numbers">Why can’t I just use numbers?</a></li>
<li class="toctree-l2"><a class="reference internal" href="#promises">Promises</a><ul>
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<div class="section" id="notes">
<h1>Notes<a class="headerlink" href="#notes" title="Permalink to this headline"></a></h1>
<p>A few quick notes about some of the less obvious aspects of interacting with
Ethereum in JavaScript.</p>
<hr class="docutils" />
<div class="section" id="why-can-t-i-just-use-numbers">
<span id="ieee754"></span><h2>Why can’t I just use numbers?<a class="headerlink" href="#why-can-t-i-just-use-numbers" title="Permalink to this headline"></a></h2>
<p>The first problem many encounter when dealing with Ethereum is the concept of numbers. Most
common currencies are broken down with very little granulairty. For example, there are only
100 cents in a single dollar. However, there are 10<sup>18</sup> <strong>wei</strong> in a single
<strong>ether</strong>.</p>
<p>JavaScript uses <a class="reference external" href="https://en.wikipedia.org/wiki/Double-precision_floating-point_format">IEEE 754 double-precision binary floating point</a> numbers to represent
numeric values. As a result, there are <em>holes</em> in the integer set after
9,007,199,254,740,991; which is problematic for <em>Ethereum</em> because that is only
around 0.009 ether (in wei).</p>
<p>To demonstrate how this may be an issue in your code, consider:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="o">&gt;</span> <span class="p">(</span><span class="n">Number</span><span class="o">.</span><span class="n">MAX_SAFE_INTEGER</span> <span class="o">+</span> <span class="mi">4</span> <span class="o">-</span> <span class="mi">5</span><span class="p">)</span> <span class="o">==</span> <span class="p">(</span><span class="n">Number</span><span class="o">.</span><span class="n">MAX_SAFE_INTEGER</span> <span class="o">-</span> <span class="mi">1</span><span class="p">)</span>
<span class="n">false</span>
</pre></div>
</div>
<p>To remedy this, all numbers (which can be large) are stored and manipulated
as <a class="reference internal" href="api-utils.html#bignumber"><span class="std std-ref">Big Numbers</span></a>.</p>
<p>The functions <a class="reference internal" href="api-utils.html#parseether"><span class="std std-ref">parseEther( etherString )</span></a> and <a class="reference internal" href="api-utils.html#formatether"><span class="std std-ref">formatEther( wei )</span></a> can be used to convert between
string representations, which are displayed to or entered by the user and Big Number representations
which can have mathematical operations handled safely.</p>
<hr class="docutils" />
</div>
<div class="section" id="promises">
<span id="promise"></span><h2>Promises<a class="headerlink" href="#promises" title="Permalink to this headline"></a></h2>
<p>A <a class="reference external" href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise">Promise in JavaScript</a> is an object which simplifies many aspects of dealing with
asynchronous functions.</p>
<p>It allows a pending result to be treated in many ways as if it has already been resolved.</p>
<p>The most useful operations you will need are:</p>
<dl class="docutils">
<dt><sup>Promise</sup> . all ( promises )</dt>
<dd>Returns a new promise that resolves once all the <em>promises</em> have resolved.</dd>
<dt><sup>prototype</sup> . then ( onResolve, onReject )</dt>
<dd><p class="first">Returns another Promise, which once the Promise was resolved, the <em>onResolve</em>
function will be executed and if an error occurs, <em>onReject</em> will be called.</p>
<p class="last">If <em>onResolve</em> returns a Promise, it will be inserted into the chain of the returned
promise. If <em>onResolve</em> throws an Error, the returned Promise will reject.</p>
</dd>
</dl>
<div class="section" id="examples">
<h3><strong>Examples</strong><a class="headerlink" href="#examples" title="Permalink to this headline"></a></h3>
<p><strong>Cleaning out an account</strong></p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">var</span> <span class="n">ethers</span> <span class="o">=</span> <span class="n">require</span><span class="p">(</span><span class="s1">&#39;ethers&#39;</span><span class="p">);</span>
<span class="n">var</span> <span class="n">targetAddress</span> <span class="o">=</span> <span class="s2">&quot;0x02F024e0882B310c6734703AB9066EdD3a10C6e0&quot;</span><span class="p">;</span>
<span class="n">var</span> <span class="n">privateKey</span> <span class="o">=</span> <span class="s2">&quot;0x0123456789012345678901234567890123456789012345678901234567890123&quot;</span><span class="p">;</span>
<span class="n">var</span> <span class="n">wallet</span> <span class="o">=</span> <span class="n">new</span> <span class="n">ethers</span><span class="o">.</span><span class="n">Wallet</span><span class="p">(</span><span class="n">privateKey</span><span class="p">);</span>
<span class="o">//</span> <span class="n">Promises</span> <span class="n">we</span> <span class="n">are</span> <span class="n">interested</span> <span class="ow">in</span>
<span class="n">var</span> <span class="n">provider</span> <span class="o">=</span> <span class="n">ethers</span><span class="o">.</span><span class="n">providers</span><span class="o">.</span><span class="n">getDefaultProvider</span><span class="p">(</span><span class="s1">&#39;ropsten&#39;</span><span class="p">);</span>
<span class="n">var</span> <span class="n">balancePromise</span> <span class="o">=</span> <span class="n">provider</span><span class="o">.</span><span class="n">getBalance</span><span class="p">(</span><span class="n">wallet</span><span class="o">.</span><span class="n">address</span><span class="p">);</span>
<span class="n">var</span> <span class="n">gasPricePromise</span> <span class="o">=</span> <span class="n">provider</span><span class="o">.</span><span class="n">getGasPrice</span><span class="p">();</span>
<span class="n">var</span> <span class="n">transactionCountPromise</span> <span class="o">=</span> <span class="n">provider</span><span class="o">.</span><span class="n">getTransactionCount</span><span class="p">(</span><span class="n">wallet</span><span class="o">.</span><span class="n">address</span><span class="p">);</span>
<span class="n">var</span> <span class="n">allPromises</span> <span class="o">=</span> <span class="n">Promise</span><span class="o">.</span><span class="n">all</span><span class="p">([</span>
<span class="n">gasPricePromise</span><span class="p">,</span>
<span class="n">balancePromise</span><span class="p">,</span>
<span class="n">transactionCountPromise</span>
<span class="p">]);</span>
<span class="n">var</span> <span class="n">sendPromise</span> <span class="o">=</span> <span class="n">allPromises</span><span class="o">.</span><span class="n">then</span><span class="p">(</span><span class="n">function</span><span class="p">(</span><span class="n">results</span><span class="p">)</span> <span class="p">{</span>
<span class="o">//</span> <span class="n">This</span> <span class="n">function</span> <span class="ow">is</span> <span class="n">ONLY</span> <span class="n">called</span> <span class="n">once</span> <span class="n">ALL</span> <span class="n">promises</span> <span class="n">are</span> <span class="n">fulfilled</span>
<span class="n">var</span> <span class="n">gasPrice</span> <span class="o">=</span> <span class="n">results</span><span class="p">[</span><span class="mi">0</span><span class="p">];</span>
<span class="n">var</span> <span class="n">balance</span> <span class="o">=</span> <span class="n">results</span><span class="p">[</span><span class="mi">1</span><span class="p">];</span>
<span class="n">var</span> <span class="n">transactionCount</span> <span class="o">=</span> <span class="n">results</span><span class="p">[</span><span class="mi">2</span><span class="p">];</span>
<span class="o">//</span> <span class="n">Sending</span> <span class="n">a</span> <span class="n">transaction</span> <span class="n">to</span> <span class="n">an</span> <span class="n">externally</span> <span class="n">owned</span> <span class="n">account</span> <span class="p">(</span><span class="n">EOA</span><span class="p">)</span> <span class="ow">is</span> <span class="mi">21000</span> <span class="n">gas</span><span class="p">)</span>
<span class="n">var</span> <span class="n">txFeeInWei</span> <span class="o">=</span> <span class="n">gasPrice</span><span class="o">.</span><span class="n">mul</span><span class="p">(</span><span class="mi">21000</span><span class="p">);</span>
<span class="o">//</span> <span class="n">This</span> <span class="n">will</span> <span class="n">send</span> <span class="n">the</span> <span class="n">maximum</span> <span class="n">amount</span> <span class="p">(</span><span class="n">our</span> <span class="n">balance</span> <span class="n">minus</span> <span class="n">the</span> <span class="n">fee</span><span class="p">)</span>
<span class="n">var</span> <span class="n">value</span> <span class="o">=</span> <span class="n">balance</span><span class="o">.</span><span class="n">sub</span><span class="p">(</span><span class="n">txFeeInWei</span><span class="p">);</span>
<span class="n">var</span> <span class="n">transaction</span> <span class="o">=</span> <span class="p">{</span>
<span class="n">to</span><span class="p">:</span> <span class="n">targetAddress</span><span class="p">,</span>
<span class="n">gasPrice</span><span class="p">:</span> <span class="n">gasPrice</span><span class="p">,</span>
<span class="n">gasLimit</span><span class="p">:</span> <span class="mi">21000</span><span class="p">,</span>
<span class="n">nonce</span><span class="p">:</span> <span class="n">transactionCount</span><span class="p">,</span>
<span class="o">//</span> <span class="n">The</span> <span class="n">amount</span> <span class="n">to</span> <span class="n">send</span>
<span class="n">value</span><span class="p">:</span> <span class="n">value</span><span class="p">,</span>
<span class="o">//</span> <span class="n">Prevent</span> <span class="n">replay</span> <span class="n">attacks</span> <span class="n">across</span> <span class="n">networks</span>
<span class="n">chainId</span><span class="p">:</span> <span class="n">provider</span><span class="o">.</span><span class="n">chainId</span><span class="p">,</span>
<span class="p">};</span>
<span class="n">var</span> <span class="n">signedTransaction</span> <span class="o">=</span> <span class="n">wallet</span><span class="o">.</span><span class="n">sign</span><span class="p">(</span><span class="n">transaction</span><span class="p">);</span>
<span class="o">//</span> <span class="n">By</span> <span class="n">returning</span> <span class="n">a</span> <span class="n">Promise</span><span class="p">,</span> <span class="n">the</span> <span class="n">sendPromise</span> <span class="n">will</span> <span class="n">resolve</span> <span class="n">once</span> <span class="n">the</span>
<span class="o">//</span> <span class="n">transaction</span> <span class="ow">is</span> <span class="n">sent</span>
<span class="k">return</span> <span class="n">provider</span><span class="o">.</span><span class="n">sendTransaction</span><span class="p">(</span><span class="n">signedTransaction</span><span class="p">);</span>
<span class="p">});</span>
<span class="n">var</span> <span class="n">minedPromise</span> <span class="o">=</span> <span class="n">sendPromise</span><span class="o">.</span><span class="n">then</span><span class="p">(</span><span class="n">function</span><span class="p">(</span><span class="n">transaction</span><span class="p">)</span> <span class="p">{</span>
<span class="o">//</span> <span class="n">This</span> <span class="n">will</span> <span class="n">be</span> <span class="n">called</span> <span class="n">once</span> <span class="n">the</span> <span class="n">transaction</span> <span class="ow">is</span> <span class="n">sent</span>
<span class="o">//</span> <span class="n">This</span> <span class="n">promise</span> <span class="n">will</span> <span class="n">be</span> <span class="n">resolve</span> <span class="n">once</span> <span class="n">the</span> <span class="n">transaction</span> <span class="n">has</span> <span class="n">been</span> <span class="n">mined</span><span class="o">.</span>
<span class="k">return</span> <span class="n">provider</span><span class="o">.</span><span class="n">waitForTransaction</span><span class="p">(</span><span class="n">transaction</span><span class="p">);</span>
<span class="p">});</span>
<span class="n">minedPromise</span><span class="o">.</span><span class="n">then</span><span class="p">(</span><span class="n">function</span><span class="p">(</span><span class="n">transaction</span><span class="p">)</span> <span class="p">{</span>
<span class="n">console</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s2">&quot;The transaction was mined: Block &quot;</span> <span class="o">+</span> <span class="n">transaction</span><span class="o">.</span><span class="n">blockNumber</span><span class="p">);</span>
<span class="p">});</span>
<span class="o">//</span> <span class="n">Promises</span> <span class="n">can</span> <span class="n">be</span> <span class="n">re</span><span class="o">-</span><span class="n">used</span> <span class="k">for</span> <span class="n">their</span> <span class="n">value</span><span class="p">;</span> <span class="n">it</span> <span class="n">will</span> <span class="ow">not</span> <span class="n">make</span> <span class="n">the</span> <span class="n">external</span>
<span class="o">//</span> <span class="n">call</span> <span class="n">again</span><span class="p">,</span> <span class="ow">and</span> <span class="n">will</span> <span class="n">provide</span> <span class="n">the</span> <span class="n">exact</span> <span class="n">same</span> <span class="n">result</span> <span class="n">every</span> <span class="n">time</span><span class="o">.</span>
<span class="n">balancePromise</span><span class="o">.</span><span class="n">then</span><span class="p">(</span><span class="n">function</span><span class="p">(</span><span class="n">balance</span><span class="p">)</span> <span class="p">{</span>
<span class="o">//</span> <span class="n">This</span> <span class="o">*</span><span class="n">may</span><span class="o">*</span> <span class="k">return</span> <span class="n">before</span> <span class="n">teh</span> <span class="n">above</span> <span class="n">allPromises</span><span class="p">,</span> <span class="n">since</span> <span class="n">it</span> <span class="n">only</span>
<span class="o">//</span> <span class="n">required</span> <span class="n">one</span> <span class="n">external</span> <span class="n">call</span><span class="o">.</span> <span class="n">Keep</span> <span class="ow">in</span> <span class="n">mind</span> <span class="n">asynchronous</span> <span class="n">calls</span> <span class="n">can</span>
<span class="o">//</span> <span class="n">be</span> <span class="n">called</span> <span class="n">out</span> <span class="n">of</span> <span class="n">order</span><span class="o">.</span>
<span class="n">console</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">balance</span><span class="p">);</span>
<span class="p">});</span>
</pre></div>
</div>
<hr class="docutils" />
</div>
</div>
<div class="section" id="checksum-address">
<span id="id1"></span><h2>Checksum Address<a class="headerlink" href="#checksum-address" title="Permalink to this headline"></a></h2>
<p>A <a class="reference external" href="https://github.com/ethereum/EIPs/issues/55">checksum address</a> uses mixed case hexidecimal strings to encode the checksum
information in the capitalization of the alphabetic characters, while remaining
backwards compatible with non-checksum addresses.</p>
<p>Example:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="o">//</span> <span class="n">Valid</span><span class="p">;</span> <span class="n">checksum</span> <span class="p">(</span><span class="n">mixed</span> <span class="n">case</span><span class="p">)</span>
<span class="mh">0xCd2a3d9f938e13Cd947eC05ABC7fe734df8DD826</span>
<span class="o">//</span> <span class="n">Valid</span><span class="p">;</span> <span class="n">NO</span> <span class="n">checksum</span> <span class="p">(</span><span class="n">no</span> <span class="n">mixed</span> <span class="n">case</span><span class="p">)</span>
<span class="mh">0xcd2a3d9f938e13cd947ec05abc7fe734df8dd826</span>
<span class="mh">0xCD2A3D9F938E13CD947EC05ABC7FE734DF8DD826</span>
<span class="o">//</span> <span class="n">INVALID</span><span class="p">;</span> <span class="p">(</span><span class="n">mixed</span> <span class="n">case</span><span class="p">,</span> <span class="n">but</span> <span class="n">case</span> <span class="n">differs</span> <span class="kn">from</span> <span class="nn">first</span> <span class="n">example</span><span class="p">)</span>
<span class="mh">0xDc2a3d9f938e13cd947ec05abc7fe734df8dd826</span>
<span class="o">^^</span>
</pre></div>
</div>
<p>To convert between ICAP and checksum addresses, see <a class="reference internal" href="api-utils.html#api-getaddress"><span class="std std-ref">getAddress()</span></a>.</p>
<hr class="docutils" />
</div>
<div class="section" id="icap-address">
<span id="id3"></span><h2>ICAP Address<a class="headerlink" href="#icap-address" title="Permalink to this headline"></a></h2>
<p>The original method of adding a checksum to an Ethereum address was by using the
a format compatible with <a class="reference external" href="https://en.wikipedia.org/wiki/International_Bank_Account_Number">IBAN</a> addresses, using the country code <strong>XE</strong>. However,
only addresses which have 0 as the first byte (i.e. the address begins with 0x00)
are truely compatible with IBAN, so ICAP extends the definition to allow for 31
alphanumeric characters (instead of the standard 30).</p>
<p>An ICAP address has the following format:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">XE</span> <span class="p">[</span><span class="mi">2</span> <span class="n">digit</span> <span class="n">checksum</span><span class="p">]</span> <span class="p">[</span><span class="n">up</span> <span class="n">to</span> <span class="mi">31</span> <span class="n">alphanumeric</span> <span class="n">characters</span><span class="p">]</span>
</pre></div>
</div>
<p>To convert between ICAP and checksum addresses, see <a class="reference internal" href="api-utils.html#api-getaddress"><span class="std std-ref">getAddress()</span></a>.</p>
<hr class="docutils" />
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