Discuss in detail the impact of lastIndex on regular results

When using regular expressions to check the same string, true and false are returned alternately. In desperation, I looked through the authoritative guide again and found that the culprit turned out to be lastIndex. This article will share with you the impact of lastIndex on regular results.

let reg = /[\d]/g
//undefined
reg.test(1)
//true
reg.test(1)
//false

lastIndex

lastIndex is explained as follows in The Definitive Guide: It is a readable/ written integer. If the matching pattern has the g modifier, this attribute is stored at the beginning of the next index in the entire string. This attribute will be used by exec() and test(). Still using the above example, observe the lastIndex attribute

##

let reg = /[\d]/g  //有修饰符g
//undefined
reg.lastIndex
//0
reg.test(1)   
//true
reg.lastIndex  //匹配一次后，lastIndex改变
//1
reg.test(1)  //从index 1 开始匹配
//false
reg.lastIndex
//0
reg.test(1)
//true
reg.lastIndex
//1

After the first successful match using test(), lastIndex is Set to the end position of matching, which is 1; when test() is performed for the second time, matching starts from index 1. If the matching fails, lastIndex is reset to 0. This causes the matching results to be inconsistent with expectations

Solution

1. Do not use the g modifier

reg = /[\d]/
///[\d]/
reg.test(1)
//true
reg.test(1)
//true
reg.lastIndex
//0
reg.test(1)
//true
reg.lastIndex

2. Manually set lastIndex = 0 after test()

The above content is the lastIndex pair The impact of regular results, I hope it can help everyone.

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