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I am using "contact form 7" WordPress plugin.

I want to disable submit button on form submit and change text like

"Submitting...." and enable on success or on error so the user can click again.

Answers

Please use this code to disable the submit button.

jQuery('.wpcf7-submit').on('click',function(){
    jQuery(this).prop("disabled",true); // disable button after clicking on button
});

As we know that the contact form 7 plugin returns various responses after submitting.

This is for mail sent event:

 document.addEventListener( 'wpcf7mailsent', function( event ) {
      jQuery(this).prop("disabled",false);// enable button after getting respone
    }, false );

see all events of contact form 7

Updated:

document.addEventListener( 'wpcf7submit', function( event ) {
    var status = event.detail.status;  
    console.log(status);  
    //if( status === 'validation_failed'){
        jQuery('.wpcf7-submit').val("Send");
    //}    
}, false );

jQuery('.wpcf7-submit').on('click',function(){
    jQuery(this).val("Submitting....");
});

Note: status returns the responses like validation_failed, mail_sent, etc after form submitted.