Here are a few title options for your article, all in the question format: * **Function Declarations vs. Expressions: Why Does Execution Order Matter in JavaScript?** * **JavaScript Timers and Functi

JavaScript Function Declaration and Evaluation Order

In JavaScript, the order of code execution can affect the behavior of a program. This is particularly relevant when dealing with function declarations and evaluations.

Consider the following example:

<code class="javascript">(function() {
  setTimeout(someFunction1, 10);
  var someFunction1 = function() { alert('here1'); };
})();</code>

This code fails with a "TypeError: someFunction1 is not a function." This is because the function declaration var someFunction1 = ... is executed after the setTimeout call, causing the variable someFunction1 to be undefined when the timeout triggers.

Contrasting this, the following code executes successfully:

<code class="javascript">(function() {
  setTimeout(someFunction2, 10);
  function someFunction2() { alert('here2'); }
})();</code>

Here, someFunction2 is a function declaration, which is processed in the compilation phase before any code execution. This ensures that someFunction2 is available when the timeout triggers.

To clarify, JavaScript code is processed in two phases:

1. Compilation: Code is parsed and syntax trees are generated. Function declarations are processed in this phase.
2. Execution: Code is interpreted and executed. Function expressions and other expressions are processed in this phase.

Function declarations are executed immediately in the compilation phase, while function expressions are evaluated at runtime in the execution phase. The order of function expressions matters because they are evaluated in the order in which they appear in the code.

In conclusion, understanding the difference between function declarations and expressions, as well as the two-phase code processing in JavaScript, is crucial for avoiding such errors.

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