


Why does `setTimeout` fail to execute with a function expression assigned to a variable before it\'s declared, but succeeds with function declarations?
JavaScript Function Declaration and Evaluation Order
Problem:
Why does the following JavaScript example fail to execute?
<code class="javascript">(function() { setTimeout(someFunction1, 10); var someFunction1 = function() { alert('here1'); }; })();</code>
While other similar examples work as intended:
<code class="javascript">(function() { setTimeout(someFunction2, 10); function someFunction2() { alert('here2'); } })(); (function() { setTimeout(function() { someFunction3(); }, 10); var someFunction3 = function() { alert('here3'); }; })(); (function() { setTimeout(function() { someFunction4(); }, 10); function someFunction4() { alert('here4'); } })();</code>
Solution:
The issue lies in the order of evaluation between function declarations and function expressions.
Function Declaration vs. Expression
- Declaration: Declares a function using the function keyword (e.g., function someFunction() {}). Function declarations are processed during the compilation phase.
- Expression: Creates a function using an expression that evaluates to a function (e.g., () => {}). Function expressions are processed during the execution phase.
Evaluation Order
JavaScript code undergoes two processing phases:
- Compilation: Variables are created and function declarations are processed.
- Execution: Function expressions and other code are executed.
Analysis of Examples
-
Example 1:
- The function someFunction1 is created as a function expression during the execution phase.
- The setTimeout function is called before someFunction1 is fully evaluated and assigned to the variable someFunction1.
- The setTimeout function receives an undefined value for someFunction1 and fails to execute.
-
Examples 2, 3, and 4:
- In each of these examples, the function is declared using the function keyword, making it a function declaration.
- Function declarations are processed during the compilation phase, so they exist before the setTimeout function is called.
- The setTimeout function receives a valid function reference and executes successfully.
Additional Clarification
- Function arguments in JavaScript are always passed by value, meaning the setTimeout function does not directly reference the variable someFunction1 but rather a copy of the function it refers to.
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