{"id":1317,"date":"2014-08-05T23:41:11","date_gmt":"2014-08-06T04:41:11","guid":{"rendered":"http:\/\/cbateman.com\/blog\/?p=1317"},"modified":"2014-10-09T22:30:51","modified_gmt":"2014-10-10T03:30:51","slug":"how-double-equals-works-in-javascript","status":"publish","type":"post","link":"https:\/\/cbateman.com\/blog\/how-double-equals-works-in-javascript\/","title":{"rendered":"How Double-Equals Works in JavaScript"},"content":{"rendered":"

tl;dr<\/strong> — Don’t use double-equals!<\/p> \r\n\r\n

When asked about the difference between double and triple-equals, a lot of JS developers will tell you that ==<\/code> compares values and ===<\/code> compares values and<\/em> types. In other words, 1 == \"1\"<\/code> will be true, and 1 === \"1\"<\/code> will be false.<\/p>\r\n\r\n

That explanation is somewhat true, but it belies a much more complicated reality.<\/p>\r\n\r\n

Some of the confusion comes from thinking that ==<\/code> is somehow related to truthiness, which is all about how variables get coerced into booleans — which happens in an if statement<\/i> like this:<\/p>\r\n\r\n

if (something) {<\/pre>\r\n\r\n
In that case — 0<\/code>, \"\"<\/code> the empty string, null<\/code>, undefined<\/code>, and NaN<\/code> will all return false. Non-zero numbers, Non-empty strings, arrays and objects (empty or not) will all return true. Most devs have a pretty good handle on this.<\/p>\r\n\r\n
But… does that mean that a non-zero number, or all non-empty strings will double-equal true? This is where things can get confusing.<\/p>\r\n\r\n
if ('a') {           \/\/ true\r\nif ('a' == true) {   \/\/ false\r\nif ('a' == false) {  \/\/ false\r\n\r\nif (2) {             \/\/ true\r\nif (2 == true) {     \/\/ false\r\nif (1 == true) {     \/\/ true\r\n<\/pre>\r\n\r\n
Hopefully that makes it clear that truthiness has nothing to do with ==<\/code>.<\/p>\r\n\r\n
Remember: truthiness is about coercion into booleans. With double-equals, nothing will ever get coerced into a boolean.<\/strong> So what’s really going on?<\/p>\r\n\r\n
The answer is the Abstract Equality Comparison Algorithm. If the two types differ, JS follows a particular process for converting them into the same type, so that it can compare them. If types don’t match somewhere along the way — the endgame will be numbers.<\/p>\r\n\r\n
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