Implementing Angular's @for Syntax for Enhanced Template Iteration

Angular 16 introduced the @for directive, a powerful tool for simplifying template iteration and conditional rendering. This challenge asks you to implement a simplified version of the @for syntax, focusing on the core iteration functionality. Successfully completing this challenge will demonstrate your understanding of Angular templates, expression evaluation, and potentially, some basic compiler-like logic.

Problem Description

You are tasked with creating a function that mimics the behavior of Angular's @for directive within a template expression. The function should take an array and an optional index variable name as input, and return a string representing the template expression that would iterate over the array. The returned string should be formatted as a valid Angular template expression, suitable for inclusion in a component's HTML.

The core functionality to implement is the generation of the for expression. The expression should include the let keyword to declare the iteration variable and the trackBy function (which you will also need to generate). For simplicity, the trackBy function will always return the index of the item.

Key Requirements:

• Array Iteration: The function must correctly generate an expression that iterates over the provided array.
• Iteration Variable: The function must include a let keyword to declare an iteration variable.
• trackBy Function: The function must include a trackBy function that returns the index of the current item.
• Valid Angular Template Expression: The generated string must be a valid Angular template expression.
• Optional Index Variable Name: The function should accept an optional second argument, which specifies the name of the index variable. If no index variable name is provided, it should default to "index".

Expected Behavior:

The function should return a string that, when placed within an Angular template, would iterate over the array and provide access to each item and its index.

Edge Cases to Consider:

• Empty Array: The function should still generate a valid expression, even if the input array is empty.
• Invalid Input: While not strictly required for this simplified version, consider how the function might handle invalid input types (e.g., a non-array input). For this challenge, assume the input is always an array.

Examples

Example 1:

Input: ['apple', 'banana', 'cherry'], 'fruit'
Output: "*for (let fruit of items; trackBy: $index)"
Explanation: The function generates an expression that iterates over the `items` array, assigning each element to the `fruit` variable. The `trackBy` function returns the index.

Example 2:

Input: [1, 2, 3]
Output: "*for (let index of items; trackBy: $index)"
Explanation: The function generates an expression that iterates over the `items` array, assigning each element to the `index` variable (default index variable name). The `trackBy` function returns the index.

Example 3: (Edge Case - Empty Array)

Input: [], 'item'
Output: "*for (let item of items; trackBy: $index)"
Explanation: Even with an empty array, the function generates a valid expression.

Constraints

• The input array can contain any type of data.
• The index variable name must be a valid JavaScript identifier (alphanumeric characters and underscores, starting with a letter).
• The function must return a string.
• The generated expression should be concise and readable.
• The function should not perform any actual iteration or rendering; it should only generate the template expression string.

Notes

• Think about how Angular's template expressions are structured.
• The trackBy function is crucial for performance in Angular templates.
• Focus on generating the correct syntax for the @for directive.
• The items variable is assumed to be the array being iterated over in the template context. Your function should generate the expression around items.
• The $ prefix is used to indicate that $index is a built-in Angular variable.