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What determines that a script is long-running?
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05/10/2009, by Nicholas C. Zakas
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One of the programming barriers that web developers are constantly bumping up against is the dreaded long-running script dialog (also called a runaway script dialog). These frightening dialogs are displayed whenever your JavaScript code takes too long to execute. The cardinal rule of web programming is that a user should never see this dialog, as it indicates a lack of proper JavaScript code architecture. Or to put it in simple terms: your code is trying to do too much.
Brendan Eich, creator of JavaScript, is quoted as saying, “[JavaScript] that executes in whole seconds is probably doing something wrong…” My personal threshold is actually much smaller: no script should take longer than 100ms to execute on any browser at any time. If it takes any longer than that, the processing must be split up into smaller chunks.
Internet Explorer determines that a script is long-running by the total amount of statements the JScript engine has executed. By default, the value is 5 million statements and can be altered via a registry setting. When your script exceeds this maximum number of statements, you’ll get this dialog:
The message, “A script on this page is causing Internet Explorer to run slowly. If it continues to run, your compute may become unresponsive.” is a bit harsh if not technically accurate. The options here are to stop the script or allow it to continue running. Script execution is completely halted while the dialog is being displayed. If you choose to continue running the script, then the executed statement count is reset. You will see the dialog again if the number of statements once again reaches the maximum.
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