Creating a variable by using a conditional operation in JavaScript

Question

Creating a variable by using a conditional operation in JavaScript

When the statement <code>name = name || {}

is used, it throws a reference error. However, using var name = name || {} works perfectly fine. Can you explain how variable initialization in JavaScript functions?

javascript

Answer 1

Answer №1

When looking at the code line:

var name = name || {};

it is important to understand that this is actually interpreted as follows:

// Upon entering the scope
var name;

// Later on, in step-by-step execution
name = name || {}

Since name is declared, it is safe to access its value (using name on the right side of the = operator). Initially, the value will be undefined, so name = name || {} will assign an empty object to it.

The significant distinction here lies in the fact that:

name = name || {}

...does not have a declaration for name, meaning that the variable does not exist when the right-hand side of the assignment is evaluated. This concept of creating global variables implicitly through assignments, as discussed in The Horror of Implicit Globals, allows you to create global variables but prevents you from reading the value of a non-existent variable.

Therefore, using var ensures proper functioning, while omitting it results in a ReferenceError.

As a side note, it is advisable to refrain from relying on implicit globals and always declare your variables explicitly (while minimizing the use of globals whenever feasible). Enabling JavaScript's "strict mode" can transform implicit globals into ReferenceErrors, thereby treating attempts to write to unknown symbols as errors instead of creating new global variables, similar to the behavior for reading undefined symbols.

Answer 2

When looking at the code line:

var name = name || {};

it is important to understand that this is actually interpreted as follows:

// Upon entering the scope
var name;

// Later on, in step-by-step execution
name = name || {}

Since name is declared, it is safe to access its value (using name on the right side of the = operator). Initially, the value will be undefined, so name = name || {} will assign an empty object to it.

The significant distinction here lies in the fact that:

name = name || {}

...does not have a declaration for name, meaning that the variable does not exist when the right-hand side of the assignment is evaluated. This concept of creating global variables implicitly through assignments, as discussed in The Horror of Implicit Globals, allows you to create global variables but prevents you from reading the value of a non-existent variable.

Therefore, using var ensures proper functioning, while omitting it results in a ReferenceError.

As a side note, it is advisable to refrain from relying on implicit globals and always declare your variables explicitly (while minimizing the use of globals whenever feasible). Enabling JavaScript's "strict mode" can transform implicit globals into ReferenceErrors, thereby treating attempts to write to unknown symbols as errors instead of creating new global variables, similar to the behavior for reading undefined symbols.

Answer 3

Answer №2

The effectiveness of the second code lies in the concept of JavaScript Hoisting.

Consider this scenario:

var person = person || {}

With hoisting, the code is rearranged as follows:

// Declaration moved to the top of the scope
var person;

// Initialization point
person = person || {};

As a result, when you initialize the variable 'person,' it has already been declared with an initial value of undefined. In contrast, the first code does not explicitly declare 'person'.

Answer 4

The effectiveness of the second code lies in the concept of JavaScript Hoisting.

Consider this scenario:

var person = person || {}

With hoisting, the code is rearranged as follows:

// Declaration moved to the top of the scope
var person;

// Initialization point
person = person || {};

As a result, when you initialize the variable 'person,' it has already been declared with an initial value of undefined. In contrast, the first code does not explicitly declare 'person'.

Answer 5

Answer №3

name = name || {} means "If the variable name already exists, set it to itself, otherwise set it to an empty object. The issue here is that you actually do not have a variable named name yet. So, when the code checks what value to assign to it, it encounters an error because it hasn't been defined.

var name = ... first declares the variable and then assigns a value to it. This approach avoids the error by creating the variable before evaluating its value. Therefore, if the variable doesn't exist, it defaults to being undefined, which can be considered falsey and set to an empty object.

This situation commonly arises in two other scenarios as well.

function a(name) { name = name || 'Joe'; }

In this function, if no argument is provided for name, it defaults to 'Joe'. Since the variable is already defined within the function argument, there is no need to use var.

defaults = { name: 'Joe' };
options = {};

// Later on
options.name = options.name || defaults.name;

Answer 6

name = name || {} means "If the variable name already exists, set it to itself, otherwise set it to an empty object. The issue here is that you actually do not have a variable named name yet. So, when the code checks what value to assign to it, it encounters an error because it hasn't been defined.

var name = ... first declares the variable and then assigns a value to it. This approach avoids the error by creating the variable before evaluating its value. Therefore, if the variable doesn't exist, it defaults to being undefined, which can be considered falsey and set to an empty object.

This situation commonly arises in two other scenarios as well.

function a(name) { name = name || 'Joe'; }

In this function, if no argument is provided for name, it defaults to 'Joe'. Since the variable is already defined within the function argument, there is no need to use var.

defaults = { name: 'Joe' };
options = {};

// Later on
options.name = options.name || defaults.name;

Answer 7

Answer №4

When utilizing the var keyword in JavaScript, it is important to declare all variables at the beginning of the scope due to a concept known as variable hoisting.

As an illustration, consider the following code snippet:

function doStuff() {
   for (var i = 0; i < arr.lenght; i++)
      var currentItem = arr[i]
      var doSomething = function(currentItem) { ... }
   }
}

JavaScript actually interprets this code in the following manner:

function doStuff() {
   var i,
       currentItem,
       doSomething;

   for (i = 0; i < arr.lenght; i++) {
      currentItem = arr[i]
      doSomething = function(currentItem) {...}
   }
}

Answer 8

When utilizing the var keyword in JavaScript, it is important to declare all variables at the beginning of the scope due to a concept known as variable hoisting.

As an illustration, consider the following code snippet:

function doStuff() {
   for (var i = 0; i < arr.lenght; i++)
      var currentItem = arr[i]
      var doSomething = function(currentItem) { ... }
   }
}

JavaScript actually interprets this code in the following manner:

function doStuff() {
   var i,
       currentItem,
       doSomething;

   for (i = 0; i < arr.lenght; i++) {
      currentItem = arr[i]
      doSomething = function(currentItem) {...}
   }
}

Creating a variable by using a conditional operation in JavaScript

Answer №1

Answer №2

Answer №3

Answer №4

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