How to set width in ngstyle using a function in angular2?

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CSS

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DOM

[ngStyle]="{'position':'absolute','top':field.y+'px','left':field.x+'px','font-size': field.fontsize,'font-family':field.fontfamily,'height':field.height+'px','width':assignWidth(field.width)+'px','border':'1px solid #FFF','background':'#EEE','overflow':'hidden' }"

What I have tried:

Here, I am getting error if I assign width using an external method(assignWidth). I want only width property in ngStyle to be assigned using a function. How to achieve this ?