NG5002: Cannot parse expression. @for loop expression must match the pattern "<identifier> of <expression>"

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04-10-2024

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NG5002: The @for Loop Mystery Solved

Angular developers often encounter the dreaded "NG5002: Cannot parse expression. @for loop expression must match the pattern ' of '" error. This error signifies a problem with the syntax of your @for loop within your Angular template. While it might seem cryptic at first glance, understanding the core issue and the proper syntax will empower you to overcome this obstacle.

The Scenario:

Let's imagine you have a component in your Angular application that displays a list of items. You want to iterate over this list using an @for loop in your template:

<ul>
  <li *ngFor="let item of items">
    {{ item.name }}
  </li>
</ul>

In this example, items is an array of objects (likely defined in your component's TypeScript file), and item is a variable representing each individual object in the array. However, let's say you accidentally wrote the loop like this:

<ul>
  <li *ngFor="item of items">
    {{ item.name }}
  </li>
</ul>

You omitted the let keyword, and Angular throws the NG5002 error.

Decoding the Error:

The NG5002 error arises because the Angular template parser requires a specific structure for @for loops. This structure is:

<identifier> of <expression>

Let's break this down:

• <identifier>: This represents a local variable that will be used to access the individual elements in the iterable. You can choose any valid JavaScript identifier for this variable.
• of: This keyword is mandatory and serves as a separator between the identifier and the iterable expression.
• <expression>: This represents the iterable object (such as an array or an object with a length property) from which the individual elements will be accessed.

The Solution:

The solution is simple: ensure your @for loop syntax follows the specified pattern.

<ul>
  <li *ngFor="let item of items">
    {{ item.name }}
  </li>
</ul>

Additional Tips:

• Understanding the let Keyword: The let keyword is crucial in the @for loop structure. It declares a local variable that acts as a placeholder for each element iterated in the loop.
• Common Mistakes:
  • Missing the let keyword: This is the most frequent cause of the NG5002 error.
  • Incorrect iterable expression: Make sure the expression you provide after of is a valid iterable.

Key Takeaways:

1. Always remember the correct syntax for @for loops in Angular templates: <identifier> of <expression>.
2. Pay close attention to the let keyword when declaring a local variable for iteration.
3. Ensure that the expression provided after of is a valid iterable object.

By understanding these fundamental principles, you can avoid the dreaded NG5002 error and build robust and efficient Angular templates.