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截取String前x位字符 ​

js
var str = '123456789'
console.log(str.slice(0,4)) //1234
console.log(str.slice(5,6)) //56
var str = '123456789'
console.log(str.slice(0,4)) //1234
console.log(str.slice(5,6)) //56

判断字符串中的字符是否都由数字组成 ​

js
function isNumeric(str) {
  return /^\d+$/.test(str);
}
console.log(isNumeric('123123')) // true
function isNumeric(str) {
  return /^\d+$/.test(str);
}
console.log(isNumeric('123123')) // true