Is JavaScript Parameter Passing By Value or By Reference?

JavaScript Parameter Passing Mechanism: By Value or By Reference?

JavaScript, like many programming languages, often raises questions about the behavior of its parameter passing mechanisms. In this case, a specific instance from a popular JavaScript reference book left readers pondering the nature of parameter passing in the language.

The Question:

The confusion arises from an example where a parameter named my appears to be undefined within a function but is later redefined. In absence of any apparent reference in the function's parameter list, readers wonder if JavaScript passes parameters by reference or by value. Additionally, the fact that no function is returned from the inner function further complicates the understanding of potential closure behavior.

The Answer:

To correctly understand JavaScript's parameter passing behavior, it's crucial to distinguish between primitive values and objects.

Primitives vs Objects:

• Primitives: Numbers, strings, and booleans behave as pass-by-value parameters. A copy of their value is passed to the function, and any modifications within the function do not affect the original value.
• Objects: Arrays and objects are passed by reference. However, this does not mean that the reference itself is passed by value. Instead, a copy of the reference is passed to the function, allowing modifications within the function to affect the original object.

In the provided example, the my parameter is a reference to an object that is defined later in the function. By passing this reference, any changes made within the function (such as adding the l and w properties) will update the original object. However, attempting to reassign the my reference itself within the function (as seen in the replace example) will not affect the caller's reference.

Closure:

The example does not demonstrate closure behavior, as no functions are returned from any other nested functions. Closure would occur only if a function defined within another function were returned, enabling it to access variables from the enclosing scope even after the enclosing function returns.

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