The HTMLObjectElement.typeMustMatch property is a Boolean that reflects the typemustmatch attribute of the <object> element and indicates if the resouce linked by it must match the MIME type given by HTMLObjectElement.type in order for this resource to be used.
var mustMatch = obj.typeMustMatch; obj.typeMustMatch = mustMatch;
<object id="obj" data="move.swf" type="application/x-shockwave-flash" typemustmatch></object>
var obj = document.getElementById("obj");
console.log(obj.typeMustMatch);
 | Specification | Status | Comment | 
|---|---|---|
| HTML Living Standard The definition of 'HTMLObjectElement' in that specification.  |  Living Standard | No change from HTML5, the latest snaspshot of the WHATWG specification. | 
| HTML5 The definition of 'HTMLObjectElement' in that specification.  |  Recommendation | The W3C specification is a latest of a previous version of HTML Living Standard First snapshot with this property.  |  
| Desktop | ||||||
|---|---|---|---|---|---|---|
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | |
| Basic support | No | No | 27 | No | No | No | 
| Mobile | |||||||
|---|---|---|---|---|---|---|---|
| Android webview | Chrome for Android | Edge Mobile | Firefox for Android | Opera for Android | iOS Safari | Samsung Internet | |
| Basic support | No | No | No | 27 | No | No | ? | 
<object>.
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    https://developer.mozilla.org/en-US/docs/Web/API/HTMLObjectElement/typeMustMatch