We are using an image as a submit button on form instead of default submit

We are using an image as a submit button on form instead of default submit button. The button is added to add custom validation script to check and compare different fieds on the form, example as to compare a date field is greater than another date field. These is done for cross field validations on the form.

The issue we are facing is that on pressing enter button on the form or any field on the form, the form bypasses the custom JS validation code and submits the form. Any idea why this happens and and which function is called internally, on pressing enter button key. Note that we do not have the default submit button.
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OriginalPostID-106954

OriginalPostID-106954