Welcome to the JavaScript feedback portal. We’re happy you’re here! If you have feedback on how to improve the JavaScript, we’d love to hear it!>
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enable spellcheck and add below codes
this.container.documentEditor.spellChecker.removeUnderline = false;
this.container.documentEditor.spellChecker.allowSpellCheckAndSuggestion = true;
this.container.documentEditor.spellChecker.enableOptimizedSpellCheck = true;
Now try to open a sfdt, using the open API.