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JavaScript JavaScript Basics (Retired) Storing and Tracking Information with Variables The Variable Challenge Solution

Sharath Chandra
Sharath Chandra
3,957 Points
Posted by Sharath Chandra
Sharath Chandra
3,957 Points

JavaScript code execution order

My question is regarding the JavaScript code execution order. In the below code, the 3 alert boxes seem to pop up first and then it is written to the document. However, the last two variables in the document.write are printed out as undefined though the input has been provided. I understand that I should first define the variables and then use them but I wanted to know how JavaScript code is executed. Is it executed sequentially? Is the JavaScript code first checked for any syntax errors and if the program has no errors, is that when the code finally runs? 

var adjective = prompt("Type the adjective word?");
document.write("There once was a " + adjective + "programmer who wanted to use JavaScript to" + verb + "the" + noun);
var verb = prompt("Type the verb?");
var noun = prompt("Type the noun?");

1 Answer

Steven Parker
Steven Parker
231,236 Points
Steven Parker
Steven Parker
231,236 Points

:bell: Hi, I was alerted by your tag.

You're right, JavaScript is executed sequentially. But before execution starts, a principle called hoisting is applied that causes function definitions and variable declarations to be handled first. Note that only the declaration of the variable is applied first, it still will not get a value until the assignment statement is executed.

Syntax errors are detected during execution and will cause the execution to stop. Lines before the error are executed.