User:Samuele2002/Ifexists.js

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new mw.Api().get( {
     action: "query",
     titles: [ "This doesn't exist", "Main Page" ],
 } ).then( function( ret ) {
     $.each( ret.query.pages, function() {
        if ( this.missing !== "" ) {
             doTheThing();
         } else {
             dontDoTheThing();
         }
     } );
 }, function( error ) {
     dontDoTheThing();
 } );