Last time we briefly covered the basics of basic datatypes, this time we’ll focus on the basics of referring to object in datatypes

A, definitions,

  • 1. Use key:value pairs (commonly known as attribute names and attribute values) to describe the characteristics of an object (every object is a complex, there are zero to multiple groups of key-value pairs);
  • 2, {key: value,… } Each group of key-value pairs is in the format of key: value, separated by commas;
  • 3. Key cannot be a reference data type and value can be any data type 
    let obj = {
	name: 'Joe'.age: 25.sex: 'male'.score: [100.98.89].fn: function () {}};console.log(obj);
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Since our objects are mostly key-value pairs, let’s talk about key-value pairs

Key value pairs

  • Composition:
    • Attribute name: Attribute value
  • Operation mode:
    • Property name = property value
    • Object [attribute name] = attribute value

    ‘dot’ and ‘[]’ can be understood here as’ of ‘

1, obtain

  • Get the value:
    • 1, object. Property name
      • Based on this method, the property name is.At the back of the
      • This way, the attribute name cannot be a number
    • 2. Object [attribute name]
      • 1. To operate in this way, ensure that the attribute name is a value (string/number/Boolean).
      • 2. If it is a variable instead of a value, it operates on the value stored in the variable as the property name of the object
      • This method can only be used if the attribute name is a number

    If the specified attribute does not exist, the obtained attribute value is undefined (no error)

    let n = 100;
let obj = {
	1: 100
};
console.log(obj[1]);
console.log(obj1.); //=>Uncaught SyntaxError: missing ) after argument list The point-based approach has its own limitations, property names cannot be numeric, cannot be objects. Numeric properties, only objects can be usedconsole.log(obj[1]);
console.log(obj['1']); //=> Other non-string formats as attribute names are no different from string formats

obj.n = 200; //=> {n:200} n is the attribute name (data format is string)
obj['n'] = 200; //=> {n:200} same as above

obj[n] = 200; //=> {100:200} => obj[100]=200   N is itself a variable (n and'n'Is a variable, is a string value), which represents the stored value100(is a number format) obj[m] =200; //=>Uncaught ReferenceError: m is not defined The variable m is not defined obj[true] = 300; //=>{true:300}
obj[undefined] = 400; //=>{undefined:400}

console.log(obj);
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  • Keys (Object) : Returns an array containing all property names 
    let obj = {
    sex: 0
};
/ / = = = = = = = = = = = = = = = = = = = = = = = = = = = =
1Gets the property value of the specified property nameconsole.log(obj.sex); / / = > 0
    console.log(obj['sex']); / / = > 0
2If the specified attribute does not exist, the obtained attribute value isundefined(No error)console.log(obj['age']); //=>undefined
3Gets all property names in the current object: returns an array containing all property namesconsole.log(Object.keys(obj)); //=>["sex"]
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2, increase | modified

  • Principle: Object attribute names (keys) are not allowed to be duplicated

    • This property is new if it did not exist before
    • If the name of the property is present, the value of the property is changed
    let obj = {
    sex: 0
};
/ / = = = = = = = = = = = = = = = = = = = = = = = = = = = =
obj.name = 'Joe';/ / = > new
obj['name'] = "Bill";//=> modify Because name: 'Zhang SAN' already exists in obj, so this operation is modified
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3, delete,

  • True delete: Removes the property from the object completely
    • delete obj.name
  • Fake delete: The current property still exists, but the property value is empty
    • obj.name = null

Reference data types cannot be used as attribute names

let n = {
    x: 100
};
let m = [100.200];
let obj = {};
obj[n] = "Zhang"; //=>obj[{x:100}] =>{"[object object]":" Zhang Three "}
obj[m] = "Bill"; //=>obj[[100,200]] =>{"100,200":" "}
console.log(obj); 
    
//=> If the attribute name must be an object, it can only be handled based on the new data structure Map in ES6
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In order to deepen our understanding, we quote three questions and explain them in detail

4. The characteristics of arrays in relation to objects

Arrays are special objects

  • 1. Its attribute name is a number. The number starts from zero and increases gradually, and each number represents the position of the current item.
  • 2. The default length property stores the length of the array
let arr = [10.20.30];
console.log(arr[0], arr[1], arr[2]);
console.log(arr.length);
console.log(arr['length']);
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Mind mapping