1. What are the features of strict mode

Variables must be declared before they are promoted. 2. Variables deleted in strict mode cannot be used. In strict mode, this points inside the function and by default points to undefind 4. In strict mode, the "this" inside the timer delayer still points to Window 5. 6. In strict mode, functions cannot be declared in block-level scopesCopy the code

2. What are the differences between var and let

Var can be used to declare the same variable repeatedly, but let can not. 2. Var has variable promotion, can be assigned first and declared later. 3. Variables declared by let have block-level scopeCopy the code

3. What are the new features of ES6 syntax

Const const 3; let const 3; Template strings 4. arrow functions 5. object and array deconstruction 6. data types Symbol 7.Set and Map collections 8.Promise object 9. Introduce the class keywordCopy the code

4. Prototype concept

Every JavaScript object contains a [proto] internal property that corresponds to its prototype function object. In addition to the prototype property, there is also a prototype property. When a function object is used as a constructor to create an instance, The prototype property value will be used as the prototype of the instance object.

5. What’s the difference between synchronous and asynchronous

Synchronization process: submit a request → wait for the server to process → Complete processing After a request is returned, subsequent operations can be processed only after the processing is completed, which is inefficient. (For example, in B/S mode) Asynchronous process: The request is submitted → the server processes the request → the server processes the request. After the request is sent, you do not need to wait for subsequent operations.Copy the code