Why Does Aliasing JavaScript Functions Cause Errors in Some Browsers But Not Others?

Aliasing JavaScript Functions: An Unexpected Hinderance

Question:

Despite attempts using both the alias and function-wrapper methods, aliasing document.getElementById fails in Firefox, Chrome, and even the debug windows. An error is thrown in Firefox, while an "Illegal invocation" occurs in Chrome. Additionally, the operation works in Internet Explorer 8.

Investigation:

After an in-depth analysis, the issue lies in the way JavaScript functions are loosely attached to objects and interact with scope.

Explanation:

When calling a JavaScript function, the interpreter assigns a scope and passes it to the function. If a function uses this to refer to the scope, the value of this will be the scope that was passed in.

However, you can manually specify the scope using the apply method, overriding the default scope assigned by the interpreter.

The Aliasing Issue:

Aliasing document.getElementById as $ encounters an error because when you call $('someElement'), the scope is set to the window object, not the document object. Consequently, if the implementation of getElementById expects the scope to be the document, the function will fail.

Fix:

To resolve this issue, you can use $.apply(document, ['someElement']) to explicitly set the scope to the document object.

Why it Works in Internet Explorer:

The unexpected success of aliasing in Internet Explorer may be attributed to the fact that in IE, the window object is equivalent to the document object. If that is the case, aliasing should function properly in IE.

Comprehensive Example:

To further illustrate the issue, consider the following Person function:

function Person(birthDate) {
  this.birthDate = birthDate;
  this.getAge = function() { return new Date().getFullYear() - this.birthDate.getFullYear(); };
  this.getAgeSmarter = function() { return this.getAge(); };
  this.getAgeSmartest = function() { return (this.constructor == Person ? this : self).getAge(); };
}

With two instances of the Person class created, yogi and anotherYogi, here are the corresponding outputs:

• console.log(yogi.getAge()): 100 (correct)
• console.log(ageAlias()): -1 (wrong scope)
• console.log(ageAlias.apply(yogi)): 100 (correct scope)
• console.log(ageSmarterAlias()): 100 (correct scope)
• console.log(ageSmarterAlias.apply(anotherYogi)): 100 (wrong scope)
• console.log(ageSmartestAlias()): 100 (correct scope)
• console.log(ageSmartestAlias.apply(document)): 100 (correct scope)
• console.log(ageSmartestAlias.apply(anotherYogi)): 200 (correct scope)

The example demonstrates the complexities of scope and the importance of using the apply method when aliasing functions.

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