Try running this code snippet in your browser:
eval("(function() { console.log(JSON.parse('" + JSON.stringify('THIS IS COOL') + "')); })()");
It will output THIS IS COOL,
however, if you run the following:
eval("(function() { console.log(JSON.parse('" + JSON.stringify('THIS IS "NOT" COOL') + "')); })()");
It will not work as expected, failing to print THIS IS "NOT" COOL
Could anyone shed light on why this occurs?
Pay attention to this detail:
The outcome of
JSON.stringify('THIS IS "NOT" COOL')"THIS IS \"NOT\" COOL"
If you attempt to parse
JSON.parse("THIS IS \"NOT\" COOL")"THIS IS \"NOT\" COOL" as "THIS IS "NOT" COOL".
On the other hand, executing
JSON.parse(JSON.stringify('THIS IS "NOT" COOL'))"THIS IS \"NOT\" COOL" is passed directly to JSON.parse.
Why does it succeed in one case, but not in the other?
I can only speculate that eval automatically unescapes any content passed to it before running the code, but I seek a definitive answer and understanding of why eval behaves in this manner.