forEach循环里break使用方式详解

let arr = [1, 2, 3, 4, 5];
arr.forEach((item, index) => {
    if (item > 2) {
        break
    }
    console.log(item)
})
// SyntaxError: Illegal break statement (非法中断语句)

let arr = [1, 2, 3, 4, 5]
for (let i = 0; i < arr.length; i++) {
    if (arr[i] > 2) {
        break // 大于2 跳出整个循环
    }
    console.log(arr[i]) // 1, 2
}

let arr = [1, 2, 3, 4, 5]
outer_block: // 标识最外层循环
for (let i = 0; i < arr.length; i++) {
    console.log('i', arr[i]) // 1
    inner_block: // 标识里面的for循环
    for (let j = 0; j < arr.length; j++) {
        if (j > 3) {
            break outer_block  // 跳出最外层循环
        }
        console.log('j', arr[j]) // 1, 2, 3, 4
    }
}

forEach(callbackfn: (value: T, index: number, array: T[]) => void, thisArg?: any): void;

Array.prototype.forEach = function(callback) {
    for (let i = 0; i < this.length; i++) {
        callback(this[i], i, this);
    } 
};

let arr = [1, 2, 3, 4, 5]
outer_block: 
for (let i = 0; i < arr.length; i++) {
    console.log('i', arr[i])
    inner_block1:
    (function() {
        inner_block2:
        break // 报错，不能直接使用break
    })()
}

function breakInForEach(arr) {
	let BreakException = {};
	let res = false;
	try {
	    arr.forEach(item => {
	       if (item === 2) {
	          res = true;
		  throw BreakException;
	        }
	    })
	}
	catch(e) {
	    if (e !== BreakException) throw e
	}
	return res;
}
console.log(foreachBreak([1， 2， 3， 4， 5， 6])); // true