var test = function() {

    'use strict';

    var mapNames = {
        'name': 'City Name:',
        'coord.lat': 'Latitute:'
    };  

    for (var key in mapNames) {

        var names;

        if (mapNames[key]) {
            name = mapNames[key];
        } else {
            name = key;
        }
    }

    console.log(name);

}

test();

In the code above I made a mistake by declaring variable names and using name instead. I thought 'strict' mode would catch it but it didn't. Shouldn't this throw an error in this case?

var test = function() {

    'use strict';

    var mapNames = {
        'name': 'City Name:',
        'coord.lat': 'Latitute:'
    };  

    for (var key in mapNames) {

        var names;

        if (mapNames[key]) {
            name = mapNames[key];
        } else {
            name = key;
        }
    }

    console.log(name);

}

test();

In the code above I made a mistake by declaring variable names and using name instead. I thought 'strict' mode would catch it but it didn't. Shouldn't this throw an error in this case?

• javascript
• strict

1 Answer 1

A name global variable already exists, unrelated to your code; it represents the name of the current window, so you are assigning to an already existing variable.

window.name; // the name of the current window for cross-window communication

Everything on window is declared as a global - so it is not reference-erroring since it is assigning to a variable in an outer scope.

Super confusing :D

• window.name on MDN.
• HTML Specification, window name property.

"use strict" would prevent defining new global variables, here we are performing an assignment to an existing variable, think of it as name is in the global scope, like window.Blob, window.console and so on.