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copied!<h2>To check if a variable (including a string) is a number, check if it is not a number:</h2> <p>This works regardless of whether the variable contains is a string or number.</p> <pre><code>isNaN(num) // returns true if the variable does NOT contain a valid number </code></pre> <h3>Examples</h3> <pre><code>isNaN(123) // false isNaN('123') // false isNaN('1e10000') // false (This translates to Infinity, which is a number) isNaN('foo') // true isNaN('10px') // true </code></pre> <p>Of course, you can negate this if you need to. For example, to implement the <code>IsNumeric</code> example you gave:</p> <pre><code>function isNumeric(num){ return !isNaN(num) } </code></pre> <h2>To convert a string containing a number into a number:</h2> <p>only works if the string <em>only</em> contains numeric characters, else it returns <code>NaN</code>.</p> <pre><code>+num // returns the numeric value of the string, or NaN // if the string isn't purely numeric characters </code></pre> <h3>Examples</h3> <pre><code>+'12' // 12 +'12.' // 12 +'12..' // Nan +'.12' // 0.12 +'..12' // Nan +'foo' // NaN +'12px' // NaN </code></pre> <h2>To convert a string loosely to a number</h2> <p>useful for converting '12px' to 12, for example:</p> <pre><code>parseInt(num) // extracts a numeric value from the // start of the string, or NaN. </code></pre> <h3>Examples</h3> <pre><code>parseInt('12') // 12 parseInt('aaa') // NaN parseInt('12px') // 12 parseInt('foo2') // NaN These last two may be different parseInt('12a5') // 12 from what you expected to see. </code></pre> <h2>Floats</h2> <p>Bear in mind that, unlike <code>+num</code>, <code>parseInt</code> (as the name suggests) will convert a float into an integer by chopping off everything following the decimal point (if you want to use <code>parseInt()</code> <em>because of</em> this behaviour, <a href="https://jsperf.com/math-floor-vs-math-round-vs-parseint/33" rel="noreferrer">you're probably better off using another method instead</a>):</p> <pre><code>+'12.345' // 12.345 parseInt(12.345) // 12 parseInt('12.345') // 12 </code></pre> <h2>Empty strings</h2> <p>Empty strings may be a little counter-intuitive. <code>+num</code> converts empty strings to zero, and <code>isNaN()</code> assumes the same:</p> <pre><code>+'' // 0 isNaN('') // false </code></pre> <p>But <code>parseInt()</code> does not agree:</p> <pre><code>parseInt('') // NaN </code></pre>

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