How to check if a variable is defined if you have a string of it in javascript?

Question

I know if you want to check if a variable a is defined you can do this

if (typeof a !== 'undefined') {
 // the variable is defined
}

but what if you want to make a function out of it, like this

function checkDefined(name) {
 return typeof name !== 'undefined';
}
checkDefined("a");

this wouldn't work, but how can I get it to work if I have to pass a string version of the variable name?

Thanks

• SLaks: You cannot do that for local variables. What problem are you trying to solve?
• Barmar: Any time you find yourself using variable variables, you should be using an object instead.
• Barmar: For global variables you can use window[name]
• omega: In my case, I only have a string of the variable name I want to check if defined or not.
• SLaks: xyproblem.info

Answer 1

Checking in global scope(window):

var a = 'test';
var b = function() {};
function checkDefined(name) {
 return typeof this[name] !== 'undefined';
}
console.log(checkDefined("a"));
console.log(checkDefined("b"));
console.log(checkDefined("c"));

If you want check if variable or function is declared in class object you should pass new context to checkDefined method:

 function MyCustomClass() {
 this.c = 'test';
 }
 function checkDefined(name) {
 return typeof this[name] !== 'undefined';
 }
 // Create new object of type MyCustomClass
 var myCustomClassObject = new MyCustomClass();
 // In this way you can check if variable/function is defined in object
 console.log(checkDefined.apply(myCustomClassObject, ["a"]));
 console.log(checkDefined.apply(myCustomClassObject, ["b"]));
 console.log(checkDefined.apply(myCustomClassObject, ["c"]));

apply will call a function immediately letting you specify both the value of this and any arguments the function will receive

• Barmar: Can you explain your answer instead of just posting code? Why use this when the function isn't an object property?
• Krzysztof Raciniewski: this is needed for checking definitions in objects. If you call this in global scope then it's refered to window, if you call this in class object - in this case MyCustomClass - it refers to MyCustomClass object context

Answer 2

Inspired by this answer. I think you can try to return with eval:

function checkDefined(name) {
 return eval("typeof " + name) !== 'undefined';
}

Example:

var a = 1;
checkDefined("a") // true
checkDefined(a) // true
checkDefined("b") // false

Answer 3

Local variables are properties of the currently scoped this object.

const log = output => document.querySelector('pre')
 .innerText += output + '\n'
 
/* In this example, running in the browser, `this` points to `window`, but 
 in other environments would still point to whatever is the global object.
 Bind `this` to the ``checkDefined` so to ensure it keeps the same value
 as where you are calling from. */
const checkDefined = function chkDef(v) {
 return typeof this[v] !== 'undefined'
}.bind(this)
a = 5
log(checkDefined('a'))
log(checkDefined('b'))
/* This is the same basic idea, but we make it a little more portable by
 not binding until right before we use it, so `this` has the correct
 scope. */
unboundCheckDefined = function chkDef(v) {
 return typeof this[v] !== 'undefined'
}
newScope()
function newScope() {
 c = 5
 const checkDefined = 
 unboundCheckDefined.bind(this)
 
 log(checkDefined('a'))
 log(checkDefined('b'))
 log(checkDefined('c'))
}

<pre></pre>

Answer 4

You should also pass on which object context you need to check if the variable is defined or not. If it's global pass the window object

function checkDefined(name, ref) {
 if(!ref) {
 ref = window;
 }
 return !!ref[name]
}
checkDefined("a"); //false if already not defined
var obj = { a: 1, b:2};
checkDefined("a",obj);//true