Web Front-end
Front-end Q&A
Share some selected web front-end interview questions worth collecting (with answers)
Share some selected web front-end interview questions worth collecting (with answers)
This article summarizes some selected Web front-end interview questions worth collecting (with answers). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.
js interview questions
-
1.js data type
- Basic data types
Number, String, Boolean, Null, Undefined, Symbol, bigInt - Reference data types
object, Array, Date, Function, RegExp
- Basic data types
-
2. Promotion of js variable and function declarations
- The declaration of variables and functions in js will be promoted to the top to execute the
- function The promotion is higher than the promotion of variables
- If an external variable with the same name is declared using var inside the function, the function will no longer search upward.
- Anonymous functions are not promoted.
-
3. Closure
- A closure is a function that can read the internal variables of other functions
- Basically, a closure It is a function that returns a function inside.
- Benefits
- You can read the variables inside the function
- Keep the variables in the memory all the time
- You can encapsulate objects Private properties and private methods
- Disadvantages
- It consumes more memory and may cause memory overflow if used improperly
-
4. The difference between == and ===
- == is equality in a non-strict sense
- Equal values are equal
- === is equality in the strict sense, and will compare the data types and value sizes of both sides
- Equal only when the value and reference address are equal
- == is equality in a non-strict sense
-
5. this
- this always points to the direct caller of the function
- If there is a new keyword, this points to the object from new
- In the event, this points to the object that triggered the event
-
6. How to traverse js arrays and objects
- for in
- for
- forEach
- for-of
- ##7. The difference between map and forEach
- The forEach method is the most basic method, which is traversal and looping. There are three parameters passed by default: the content of the traversed array
- , the array index index, and the current traversed array Array
map method, the basic usage is the same as forEach, but different, it will return a new array, so callback - needs to have a return value, if not, it will return undefined
- , the array index index, and the current traversed array Array
- 8. What is the difference between arrow functions and ordinary functions?
- The this object in the function body is the object where it is defined, not the object where it is used
- It cannot be used as a constructor, that is, the new command cannot be used, otherwise an error will be thrown
- The arguments object cannot be used, and the object does not exist in the function body. If you want to use it, you can use the Rest parameter instead
- The yield command cannot be used, so the arrow function cannot be used as the Generator function
- 9. Same-origin policy
- The same origin refers to the same domain name, protocol, and port number
- 10. How to solve cross-domain issues
- jsonp cross-domain
- document.domain iframe cross-domain
- nodejs middleware proxy cross-domain
- The backend sets the secure domain name in the header information
- 11. Restrictions of strict mode
- Variables must be declared before use
- The parameters of the function cannot have the same attribute, otherwise an error will be reported
- Cannot use the with statement
- Prohibit this from pointing to the global object
- 12, es6 new addition
- New template string
- Arrow function
- for-of (used to traverse data—such as values in an array.)
- ES6 incorporates Promise objects into the specification and provides native Promise objects.
- Added let and const commands for declaring variables.
- There is also the introduction of the concept of module module
- 13. What is the difference between attribute and property?
- Attribute is The properties
- property that the dom element has as an html tag in the document are the properties that the dom element has as an object in js.
- For the standard attributes of html, attribute and property are synchronized and will be updated automatically
- But for custom attributes, they are not synchronized
- 14. What is the difference between let and const?
- The let command does not have variable promotion. If used before let, an error will be reported
- If there are let and const commands in the block area, a closed scope will be formed
- Repeated declarations are not allowed
- const defines a constant and cannot be modified, but if it defines an object, The data inside the object can be modified
- 15. Memory leak
- Definition: The heap memory that has been dynamically allocated in the program has not been allocated for some reason. Various problems caused by release or inability to release.
- Possible memory leaks in js: Result: slowdown, crash, large delay, etc.
- Possible causes of memory leaks in js
- Global variables
- When the dom is cleared, there are still references
- The timer is not cleared
- Memory leak caused by the existence of sub-elements
- 16. How to introduce script?
html static<script> introduction</script>js dynamic insertion<script></script>- ##
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