JavaScript条件空字符串

Question

I would like to validate a date (passed in arg as a string formatted like 99/99/9999). For my needs, empty/null/undefined string should be considered as good.

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Here is my validation function:

function isDateValid(date) {
    if(date && date.length == 0){
        return true;
    }
    //more checks
}

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I got strange results, that I can't explain:

• With a empty string (string with 0 char length) the condition is evaluated to an empty String and therefore don't reach the return true

• With a filled string (ex: "01/01/2018") the condition is evaluated to a false boolean.

I know how to change to make the code run as I need.... but this makes me sick to understand why this has 2 different comportement (once string, once boolean).

I need a captain here!

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Adding some console.log as

function isDateValid(date) {
    console.log(date);
    console.log(date.length);
    console.log(date && date.length == 0);
    console.log(typeof (date && (date.length == 0)));
    if(date && date.length == 0){
        return true;
    }
    //more...
}

will produce the following output:

Here is a fiddle https://jsfiddle.net/v8easudj/2/

Answer 1

JavaScript has a notion of truthy and falsy coercion so "" (empty string), null and undefined (as well as a few others) are falsey, so

if (!date)
    return true;

Will simultaneously check if date is null, undefined or an empty string and return true.

For more information check out the articles on MDN

• Truthy
• Falsy
• Type Conversion/Coercion

Answer 2

As other answers stated, JavaScript has a notion of truthy and falsy values -- ie values that are implicitly considered true or false when you use them as booleans (like in if statements or with conditional operators).

In particular, empty strings are falsy.

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In your case, you get a different typeof , because the && boolean operator gets short-circuited on a falsy value. It stops further evaluation and returns the left operand.

So you feed in an empty string, && stops there and you get the typeof of what's on the left -- ie of the empty string.

You feed in a non-empty string, && proceeds and you get the typeof of its right operand -- ie of the boolean result of date.length == 0 .