What exactly is the functionality of this "if" statement that operates without any operators?

I've recently started learning Javascript and I'm currently going through chapter 5 of Eloquent Javascript. In my studies, I encountered a piece of code that is puzzling to me. While I understand the general method and steps of how it works, I'm struggling to grasp why it works.

Here's the code snippet in question:

function filter(array, test) {
  var passed = [];
  for (var i = 0; i < array.length; i++) {
    if (test(array[i]))
      passed.push(array[i]);
  }
  return passed;
}

Essentially, the function takes each element being iterated over by the 'for loop' from the array and compares it to the test parameter.

My confusion lies with this part of the code:

if (test(array[i]))

There are no obvious comparison operators such as || or &&. How does it compare values using just parentheses?

How exactly does test get compared to the value of array[i] without any explicit operators?

If you'd like to access the file, here's the link: . Navigate to the 'Filtering an Array' exercise.

Thank you!

Answer №1

In an if statement, whatever is inside the parentheses will be examined for truthiness or falsiness. If the result is a falsy value (false, 0, "", NaN, null, undefined), the evaluation fails. Conversely, if the result is a truthy value (anything else), the evaluation passes. For instance, if your if statement includes a function call like if (test(something)) {}, then it essentially means

if the outcome of the function call is truthy, proceed
.

It's important to note that && and || are not comparison operators; they are, in fact, boolean operators. These operators serve to connect the results of two expressions, such as true || false which results in true.

Answer №2

It appears to be a custom function, possibly involving some sort of comparison that yields a True or False result. Providing the complete code would enable us to offer a more detailed explanation.

Answer №3

This snippet of code is designed to accept a parameter labeled as `test`, which is classified as a "predicate function". This type of function evaluates an element and returns either true or false based on the evaluation.

One practical example of how this code can be utilized is demonstrated below:

var large_numbers = filter(numbers, function(x){ return x > 100; });

Essentially, the expected input for the `test` parameter in this scenario is essentially a block of code.

In the realm of Javascript, the practice of passing around code snippets is not only common but also considered highly idiomatic. This contrasts with other programming languages that lack support for closures and nested functions, thereby restricting all code to exist at the top level without any associated context (e.g., C language).

Answer №4

The term 'test' represents a function rather than a specific value. In the world of JavaScript, functions are considered objects and can be utilized as parameters. In this particular scenario, the test function is designed to accept one parameter and provide either a true or false outcome based on that input.

Within the context of a for loop, the test function is invoked with each element of an array. If the result happens to be true, it will then be stored in a separate array. Ultimately, the elements meeting the criteria will be returned back to the calling function.

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