Is there a standard function to check for null, undefined, or blank variables in JavaScript?

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You can just check if the variable has a truthy value or not. That means

if( value ) {
}

will evaluate to true if value is not:

• null
• undefined
• NaN
• empty string ("")
• 0
• false

The above list represents all possible falsy values in ECMA-/Javascript. Find it in the specificationat the ToBoolean section.

Furthermore, if you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. For instance

if( typeof foo !== ‘undefined‘ ) {
    // foo could get resolved and it‘s defined
}

If you can be sure that a variable is declared at least, you should directly check if it has a truthyvalue like shown above.

Further read: http://typeofnan.blogspot.com/2011/01/typeof-is-fast.html

http://stackoverflow.com/questions/5515310/is-there-a-standard-function-to-check-for-null-undefined-or-blank-variables-in/5515349#5515349

Is there a standard function to check for null, undefined, or blank variables in JavaScript?

标签：als   tor   define   res   .com   logs   stack   blog   typeof   

原文地址：http://www.cnblogs.com/chengdabelief/p/6759679.html