2-13 Single thread, Synchronous execution

bigword alert : single thread

• one command at a time
• under the hood of the brower, maybe not

bigwod alert : synchronous

• one at a time, and in order

講師霸氣的說:現在先相信我，js是 single thread, synchronous

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2-12 Code execution

Recall : two phases to run a js code

1. creation phase : set up global object , setup variables, outer environments …
2. execution phase : execution the code “line by line”

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2-11 Undefined

千萬不要自己將變數的值設成undefined，以保證undefined都是js engine 所設的，方便debug

作者覺得叫notset 比較合適 😤

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console.log(a);

//Uncaught ReferenceError: a is not defined

• a does not exist in memory

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var a;

console.log(a);

//undefined

• a exists in memory, but is set to a special value - undefined

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var a;

if (a === undefined){
  console.log("a is not defined");
}
else{
  console.log("a is definded");
}

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2-10 Creation and hoisting

• hoisting : before js code is run line by line, js engine set memory space for the variables
• all variables is initialized to undefined in js

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b();
console.log(a);

var a = 'Bello';
function b (){
  console.log('called b');
}
/*
called b
undefined
 */

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b();
console.log(a);

function b (){
  console.log('called b');
}

/*
called b
Error!!
*/

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