Determine if conversion from string to 32-bit integer will overflow

Question

Trying to do front-end validation on an HTML input instead of throwing an exception in the Java back-end.

SLaks · Accepted Answer · 2010-02-17 16:35:56Z

2

Check whether the number is more than 2147483647.

For example:

if (parseInt(num, 10) > 2147483647)
    //BAD!!!

answered Feb 17, 2010 at 16:35

SLaks

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3 Comments

Jonathon Faust Over a year ago

Good -- might be worth mentioning that you must do input validation on the Java back end as well as the front end.

kennytm Over a year ago

@SLaks: Perhaps you need to check the lower bound (-2147483648) as well.

user267272 Over a year ago

it is signed, so all of the above apply. why didn't I think of this?

kennytm · Accepted Answer · 2010-02-17 16:36:46Z

1

if ((the_number >> 0) != the_number) {
  // overflow...
}

You still need a server-side check because the client-side may turn off Javascript, etc.

answered Feb 17, 2010 at 16:36

kennytm

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3 Comments

Jonathon Faust Over a year ago

@KennyTM - if someone has only worked with JavaScript and Java, they might not be familiar with lower level operations like bit shifts -- could you flesh out an explanation of what you're doing? OP doesn't know enough to work out what 32-bit overflow would look like, so an explanation would probably help.

user267272 Over a year ago

I am familiar with 32-bit overflow. does javascript explicitly use 32-bit integers, or is this open to implementation?

kennytm Over a year ago

@user: 11.7.2 The Signed Right Shift Operator: "The result is a signed 32-bit integer." (Ref: ecma-international.org/publications/files/ECMA-ST/ECMA-262.pdf)

Phill · Accepted Answer · 2010-02-17 16:39:42Z

0

Just check:

if (parseInt(myNumberAsString, 10) > 2147483647) {  alert("Invalid int!"); }

answered Feb 17, 2010 at 16:39

Phill

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