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
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
this when the function isn't an object property?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 contextInspired 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
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>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
window[name]