What is js garbage collection mechanism? Introduction to js garbage collection mechanism

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JS has an automatic garbage collection mechanism. The garbage collector is executed periodically at fixed intervals.

Common garbage collection methods in Js: mark clearing and reference counting.

1. Mark clearing method:

Working principle: When a variable enters the environment, mark the variable as "entering the environment". When a variable leaves the environment, it is marked as "leaving the environment". The memory marked "leaving the environment" is recycled.

Workflow:

• The garbage collector will mark all variables stored in memory during operation;

• Remove the tags of variables in the environment and variables referenced by variables in the environment;

• is marked will be treated as variables ready for deletion;

• The garbage collector completes the memory cleanup work, destroys those marked values ​​and reclaims the memory space they occupy. .

2. Reference counting method:

Working principle: Track and record the number of times each value is referenced.

Work flow:

• Declare a variable and assign a reference type value to the variable. The number of references to this reference type value is 1;

• The same value is assigned to another variable, and the number of references of this reference type value is increased by 1;

• When the variable containing this reference type value is assigned another value, then the number of references to this reference type value is reduced by 1;

• When the number of references becomes 0, it means that there is no way to access this value.

• #When the garbage collector runs next time, it will release the memory occupied by the value with a reference count of 0.

But when there is a circular reference, the memory cannot be released - causing a memory leak

Because the implementation of BOM and DOM in IE uses COM, and COM objects The garbage collection mechanism used is the reference technology strategy. So there will be a circular reference problem.

Solution: Manually disconnect the link between the JS object and the DOM and assign the value to null. IE9 converts DOM and BOM into real JS objects, so this problem is avoided.

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