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    <p>All arrays in JavaScript contain <code>array.length</code> elements, starting with <code>array[0]</code> up until <code>array[array.length - 1]</code>. By definition, an array element with index <code>i</code> is said to be part of the array if <code>i</code> is between <code>0</code> and <code>array.length - 1</code> inclusive.</p> <p>That is, JavaScript arrays are linear, starting with zero and going to a maximum, and arrays don't have a mechanism for excluding certain values or ranges from the array. To find out if a value exists at a given position index (where index is 0 or a positive integer), you literally just use</p> <pre><code>if (index &lt; array.length) { // do stuff } </code></pre> <p>However, it <em>is</em> possible for some array values to be null, <code>undefined</code>, <code>NaN</code>, <code>Infinity</code>, 0, or a whole host of different values. For example, if you add array values by increasing the <code>array.length</code> property, any new values will be <code>undefined</code>. </p> <p>To determine if a given value is something meaningful, or has been defined. That is, <strong>not</strong> <code>undefined</code>, or <code>null</code>:</p> <pre><code>if (typeof array[index] !== 'undefined') { </code></pre> <p>or</p> <pre><code>if (typeof array[index] !== 'undefined' &amp;&amp; array[index] !== null) { </code></pre> <p>Interestingly, because of JavaScript's comparison rules, my last example can be optimised down to:</p> <pre><code>if (array[index] != null) { // The == and != operator consider null equal to only null or undefined </code></pre>
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    1. POHow do I check in JavaScript if a value exists at a certain array index?
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    1. USthomasrutter
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    1. POHow do I check in JavaScript if a value exists at a certain array index?
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    1. COIt's highly likely that the OP knows what sort of array s/he's dealing with, but just for completeness: *array-like* objects also typically contain a `length` property, in which case the later two examples are more appropriate.
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      1. USJustin Johnson
    2. COThanks Justin .... I am actually quite green between the ears about JS, so this is all helpful info
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      1. USAnkur
    3. COThe top example is also applicable to array-like objects. The definition of an array-like object is an object with property `obj.length`, and properties numbered `0` to *obj.length-1*.
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