javascript - NodeJS, mysql, and async results -

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• javascript closure inside loops – simple practical example 31 answers

i'm using nodejs truncate list of 60 tables in database.

to this, i'm using code:

var mysql      = require('mysql');  var connexion = mysql.createconnection({   host     : '1.1.1.1',   user     : 'user',   password : 'password',   database : 'db' }); connexion.connect(); var tablelist = [     'table0',     'table1',     'table2',     ... ]; console.log('\n### truncating tables... ###'); var i; for(i = 0; < tablelist.length; i++){     connexion.query('truncate '+tablelist[i]+';', function(err, rows, fields){         if(err) throw err;         console.log(i+'===> '+tablelist[i]+' truncated.')     });  } 

the problem output always:

60===> undefined truncated. 60===> undefined truncated. 60===> undefined truncated. 60===> undefined truncated. 60===> undefined truncated. 60===> undefined truncated. 

i have 2 problems, tablelist array not accessible within callback function, , callback function keeps thinking i=60 correct value.

what doing wrong?

i have like:

0===> table0 truncated. 1===> table1 truncated. ... 

classic - async + loops not mix well.

you variable i. loops 0 tablelist.length - 1, , stops @ tablelist.length. each one, launch async request, callback.

now callbacks start hitting you, i tablelist.length, , tablelist[i] not exist. (tablelist accessible, being under closure; you're hitting past end due i having whatever value got left after loop ended.)

how solve? wrap contents of loop in iife, passing in loop counter (or values depending on loop counter) no bad effects closure closing on wrong thing:

for (i = ....) {   (function(i) {     ...   })(i); }