JavaScript: The Definitive Guide, Sixth Editio javaScript权威指南(第6版) pdf 文字版-文字版, javascript电子书, 和javascript 有关的电子书:

3.7 Immutable Primitive Values and Mutable Object References

There is a fundamental difference in JavaScript between primitive values (undefined,
null, booleans, numbers, and strings) and objects (including arrays and functions).
Primitives are immutable: there is no way to change (or “mutate”) a primitive value.
This is obvious for numbers and booleans—it doesn’t even make sense to change the
value of a number. It is not so obvious for strings, however. Since strings are like arrays
of characters, you might expect to be able to alter the character at any specified index.
In fact, JavaScript does not allow this, and all string methods that appear to return a
modified string are, in fact, returning a new string value. For example:
var s = "hello";   // Start with some lowercase text
s.toUpperCase();   // Returns "HELLO", but doesn't alter s
s                  // => "hello": the original string has not changed
Primitives are also compared by value: two values are the same only if they have the
same value. This sounds circular for numbers, booleans, null, and undefined: there is
no other way that they could be compared. Again, however, it is not so obvious for
strings. If two distinct string values are compared, JavaScript treats them as equal if,
and only if, they have the same length and if the character at each index is the same.
Objects are different than primitives. First, they are mutable—their values can change:
var o = { x:1 };     // Start with an object
o.x = 2;             // Mutate it by changing the value of a property
o.y = 3;             // Mutate it again by adding a new property
var a = [1,2,3]      // Arrays are also mutable
a[0] = 0;            // Change the value of an array element

a[3] = 4;            // Add a new array element

Objects are not compared by value: two objects are not equal even if they have the same
properties and values. And two arrays are not equal even if they have the same elements
in the same order:
var o = {x:1}, p = {x:1};  // Two objects with the same properties
o === p                    // => false: distinct objects are never equal
var a = [], b = [];        // Two distinct, empty arrays
a === b                    // => false: distinct arrays are never equal
Objects are sometimes called  reference types to distinguish them from JavaScript’s
primitive types. Using this terminology, object values are references, and we say that
objects are compared by reference: two object values are the same if and only if they
refer to the same underlying object.
var a = [];   // The variable a refers to an empty array.
var b = a;    // Now b refers to the same array.
b[0] = 1;     // Mutate the array referred to by variable b.
a[0]          // => 1: the change is also visible through variable a.
a === b       // => true: a and b refer to the same object, so they are equal.
As you can see from the code above, assigning an object (or array) to a variable simply
assigns the reference: it does not create a new copy of the object. If you want to make
a new copy of an object or array, you must explicitly copy the properties of the object
or the elements of the array. This example demonstrates using a for loop (§5.5.3):
var a = ['a','b','c'];              // An array we want to copy
var b = [];                         // A distinct array we'll copy into
for(var i = 0; i < a.length; i++) { // For each index of a[]
    b[i] = a[i];                    // Copy an element of a into b
}
Similarly, if we want to compare two distinct objects or arrays, we must compare their
properties or elements. This code defines a function to compare two arrays:
function equalArrays(a,b) {
    if (a.length != b.length) return false; // Different-size arrays not equal
    for(var i = 0; i < a.length; i++)       // Loop through all elements
        if (a[i] !== b[i]) return false;    // If any differ, arrays not equal
    return true;                            // Otherwise they are equal
}

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