Function Declarations within Conditional Statements

In JavaScript, function declarations have a different behavior depending on the prevailing language standard and the environment in which they are executed.

In Strict Mode (ES5)

In strict mode, introduced in ECMAScript 5 (ES5), function declarations are not allowed within conditional statements. This is because function declarations create hoisted variables, which are scoped to the entire function or global scope. Placing them within a conditional statement would break this hoisting mechanism.

Non-Strict Mode (ES5)

In non-strict mode, however, the behavior of function declarations within conditional statements was unpredictable. Different browsers and JavaScript engines implemented their own rules for handling this situation, leading to inconsistent results.

In Modern JavaScript (ES2015)

As of 2018, most modern browsers support ECMAScript 2015 (ES2015), which introduced a stricter interpretation of function declarations within blocks. In ES2015, function declarations are scoped to the block in which they are declared.

Example:

Consider the following code:

var abc = '';
if (1 === 0) {
  function a() {
    abc = 7;
  }
} else if ('a' === 'a') {
  function a() {
    abc = 19;
  }
} else if ('foo' === 'bar') {
  function a() {
    abc = 'foo';
  }
}
a();
document.write(abc); //writes "foo" even though 'foo' !== 'bar'

In strict mode or in ES2015, this code would result in an error because the function a is not defined in the global scope. However, in non-strict mode, it might produce different outputs depending on the implementation. In the example provided, Chrome outputs "foo" while Firefox outputs "19".

Recommendation:

To avoid unexpected behavior, it is recommended to use function expressions instead of function declarations when conditionally defining functions. Function expressions create scoped functions that are only accessible within their immediate scope.