What is the full form of an expressionless statement in javascript?

2018-06-07 09:26:12

This question already has an answer here:

Detecting an undefined object property 40 answers

if (foo) { 
}

"What is this equivalent to?"

It's not equivalent to any of the ones you suggested. It would be equivalent to:

if (Boolean(foo)) { }

or the same thing by using the ! operator:

if (!!foo) { }

or you could be explicit in your comparison if you really want.

if (!!foo === true) { }

"My actual motivation for asking is wanting to ensure that a member "exists"..."

To find if a member exists in an object, use the in operator.

if ("devicePixelRatio" in window) { }

"... (that is to say, if it is null or undefined then it doesn't exist):"

To check for not null or undefined , which is no the same as not existing, do this:

if (window.devicePixelRatio != null) { }

The != operator will perform both a null and undefined check at the same time. Any other value will meet the condition.

链接地址: http://www.djcxy.com/p/22642.html