Why is let=0 valid but not var=0? : <p>Why doesn't</p>

<p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
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<pre class="snippet-code-js lang-js prettyprint-override"><code>let=0</code></pre>
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<p>show any syntax errors but</p>

<p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
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<pre class="snippet-code-js lang-js prettyprint-override"><code>var=0</code></pre>
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<p>does? (I test it on Safari)</p>

<p>However I tried</p>

<p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
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<pre class="snippet-code-js lang-js prettyprint-override"><code>console.log(let)</code></pre>
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<p>but it has errors and seems 'let' is not a already defined variable. Why would that happen?</p>
