JavaScript syntax
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Function declarations, which declare a variable and assign a function to it, are similar to variable statements, but in addition to hoisting the declaration, they also hoist the assignment – as if the entire statement appeared at the top of the containing function – and thus forward reference is also possible: the location of a function statement within an enclosing function is irrelevant. This is different from a function expression being assigned to a variable in a var, let, or const statement.
So, for example,
var func = function() { .. } // declaration is hoisted only
function func() { .. } // declaration and assignment are hoisted
Block scoping can be produced by wrapping the entire block in a function and then executing it – this is known as the immediately-invoked function expression pattern – or by declaring the variable using the let keyword.
Declaration and assignment
Variables declared outside a scope are global. If a variable is declared in a higher scope, it can be accessed by child scopes.
When JavaScript tries to resolve an identifier, it looks in the local scope. If this identifier is not found, it looks in the next outer scope, and so on along the scope chain until it reaches the global scope where global variables reside. If it is still not found, JavaScript will raise a ReferenceError exception.
When assigning an identifier, JavaScript goes through exactly the same process to retrieve this identifier, except that if it is not found in the global scope, it will create the "variable" in the scope where it was created.cite-ref-9[9] As a consequence, a variable never declared will be global, if assigned. Declaring a variable (with the keyword var) in the global scope (i.e. outside of any function body (or block in the case of let/const)), assigning a never declared identifier or adding a property to the global object (usually window) will also create a new global variable.
Note that JavaScript's strict mode forbids the assignment of an undeclared variable, which avoids global namespace pollution.
Examples
Here are some examples of variable declarations and scope:
var x1 = 0; // A global variable, because it is not in any function
let x2 = 0; // Also global, this time because it is not in any block
function f() {
var z = 'foxes', r = 'birds'; // 2 local variables
m = 'fish'; // global, because it was not declared anywhere before
function child() {
var r = 'monkeys'; // This variable is local and does not affect the "birds" r of the parent function.
z = 'penguins'; // Closure: Child function is able to access the variables of the parent function.
}
twenty = 20; // This variable is declared on the next line, but usable anywhere in the function, even before, as here
var twenty;
child();
return x1 + x2; // We can use x1 and x2 here, because they are global
}
f();
console.log(z); // This line will raise a ReferenceError exception, because the value of z is no longer available
for (let i = 0; i < 10; i++) console.log(i);
console.log(i); // throws a ReferenceError: i is not defined
for (const i = 0; i < 10; i++) console.log(i); // throws a TypeError: Assignment to constant variable
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