javascript set a variable if undefined

问题

I know that I can test for a javascript variable and then define it if it is undefined, but is there not some way of saying

var setVariable = localStorage.getItem('value') || 0;

seems like a much clearer way, and I'm pretty sure I've seen this in other languages.

回答1:

Yes, it can do that, but strictly speaking that will assign the default value if the retrieved value is falsey, as opposed to truly undefined. It would therefore not only match undefined but also null, false, 0, NaN, "" (but not "0").

If you want to set to default only if the variable is strictly undefined then the safest way is to write:

var x = (typeof x === 'undefined') ? def_val : x;

On newer browsers it's actually safe to write:

var x = (x === undefined) ? def_val : x;

but be aware that it is possible to subvert this on older browsers where it was permitted to declare a variable named undefined that has a defined value, causing the test to fail.

回答2:

The 2018 ES6 answer is:

return Object.is(x, undefined) ? y : x;

If variable x is undefined, return variable y... otherwise if variable x is defined, return variable x.

回答3:

It seems more logical to check typeof instead of undefined? I assume you expect a number as you set the var to 0 when undefined:

var getVariable = localStorage.getItem('value');
var setVariable = (typeof getVariable == 'number') ? getVariable : 0;

In this case if getVariable is not a number (string, object, whatever), setVariable is set to 0

回答4:

I needed to "set a variable if undefined" in several places. I created a function using @Alnitak answer. Hopefully it helps someone.

function setDefaultVal(value, defaultValue){
   return (value === undefined) ? defaultValue : value;
}

Usage:

hasPoints = setDefaultVal(this.hasPoints, true);

回答5:

If you're a FP fan, Ramda has a neat helper function for this called defaultTo :

usage:

const result = defaultTo(30)(value)

It's more useful when dealing with undefined boolean values:

const result2 = defaultTo(false)(dashboard.someValue)

回答6:

var setVariable = (typeof localStorage.getItem('value') !== 'undefined' && localStorage.getItem('value')) || 0;

回答7:

Ran into this scenario today as well where I didn't want zero to be overwritten for several values. We have a file with some common utility methods for scenarios like this. Here's what I added to handle the scenario and be flexible.

function getIfNotSet(value, newValue, overwriteNull, overwriteZero) {
    if (typeof (value) === 'undefined') {
        return newValue;
    } else if (value === null && overwriteNull === true) {
        return newValue;
    } else if (value === 0 && overwriteZero === true) {
        return newValue;
    } else {
        return value;
    }
}

It can then be called with the last two parameters being optional if I want to only set for undefined values or also overwrite null or 0 values. Here's an example of a call to it that will set the ID to -1 if the ID is undefined or null, but wont overwrite a 0 value.

data.ID = Util.getIfNotSet(data.ID, -1, true);

回答8:

Works even if the default value is a boolean value:

var setVariable = ( (b = 0) => b )( localStorage.getItem('value') );

回答9:

It seems to me, that for current javascript implementations,

var [result='default']=[possiblyUndefinedValue]

is a nice way to do this (using object deconstruction).

来源：https://stackoverflow.com/questions/5409641/javascript-set-a-variable-if-undefined

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