In JavaScript there is two different standards for the codes. One is the non-strict mod, and another is the strict mode. The strict mod limits the scope of a variable or a function to the block they nest in, while the non-strict mode doesn’t.

Why should there be two different mod?
Because the non-strict mode is the initial style when JS is newly created. And upon it is widely used, the problem of non-strict style imerged. So developments need to change it, but there are still so many old websites using it, if the non-strict mode is totally abandoned, the old websites will no longer work.
So it comes as an optional mode.

We could use the strict mode in node.js by adding a 'use strict'; line in the js file’s at start.

Function Expression

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  • functions are values. They can be assigned, copied or declared in any place of the code.
  • if the function is declared as a separate statement in the main code flow, that’s called a “Function Declaration”
  • if the function is created as a part of an expression, it’s called a “Function Expression”
  • Function Declarations are processed before the code block is executed. They are visible everywhere in the block.
  • Function Expressions are created when the execution flow reaches them.

Arrow Function

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Arrow functions are handy for simple actions, especially for one-liners. They come in two flavors:

  • Without curly braces: (…args) => expression – the right side is an expression: the function evaluates it and returns the result. Parentheses can be omitted, if there’s only a single argument, e.g. n => n*2.

  • With curly braces: (…args) => { body } – brackets allow us to write multiple statements inside the function, but we need an explicit return to return something.

eg: Replace Function Expressions with arrow functions in the code below:

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function ask(question, yes, no) {
    if (confirm(question)) yes();
    else no();
}

ask(
    "Do you agree?",
    function() { alert("You agreed."); },
    function() { alert("You canceled the execution"); }
);

Let’s change it to arrow function expressions:

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ask = (question, yes, no) => {
    if (confirm(question)) yes();
    else no();
    return 0;
}

JavaScript Call Stack

  • JavaScript engine uses a call stack to manage execution contexts.

  • The call stack uses the stack data structure that works based on the LIFO (last-in-first out) prinsiple.