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for(var i=0; i<4; i++) {
    setTimeout(function() {
    console.log('setTimeout: ', i);
    }, 0);
    console.log(i);
}
/*
 * output
 * 0
 * 1
 * 2
 * 3
 * setTimeout: 4
 * setTimeout: 4
 * setTimeout: 4
 * setTimeout: 4
 */
 
 for(var i=0; i<4; i++) {
    (function(len){
        setTimeout(function(){
            console.log('function: ', len);
        },0)
    })(i);
    console.log(i);
}
/*
 * output
 * 0
 * 1
 * 2
 * 3
 * function: 0
 * function: 1
 * function: 2
 * function: 3
 */

var len=4; while(len--){ setTimeout(function(){ alert(len); },0); alert(len); }
// output: 3，2，1，0，-1，-1，-1，-1.

var len=4; while(len--){ (function(i){ setTimeout(function(){ alert(i); },0); })(len); alert(len); }
// output: 3,2,1,0,1,2,3,0