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I'm attempting to call a javascript method in its own file from code behind on a button click.

aspx file

protected void Next_Click(object sender, EventArgs e)
{
    if (hidden.Value == "")
    {
        Response.Write(@"<script language='javascript'>drawImage();</script>");
    }
}

js file

function drawImage() {
    context.drawImage(video, 0, 0, 320, 240);
    var imgBase = canvas.toDataURL('image/jpeg');
    document.getElementById("hidden").value = imgBase;
}

The problem is that this wont call the draw image method, i suspect it's cause the js file is its own file but i'm not sure.

Any help you could give would be greatly appreciated.

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2 Answers

up vote 0 down vote

1). If the function is stored in external file (e.g. MyScript.js) then you should include it in the <head> section of the web page and add onclick event to the button, like:

ButtonName.Attributes.Add("onclick", {YourFunction});

in Page_Load event.

2) Another approach is to assign the entire javascript function to a string variable and then pass it instead of {YourFunction} prefixed with javascript:. In this case you don't even need the external file.

Hope it will help. My best, Alex

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up vote 0 down vote

I presume you already included the script file in your page.

If you require the postback, you need ClientScriptManager.RegisterStartupScript Method (Type, String, String, Boolean) .. 

protected void Next_Click(object sender, EventArgs e)
{
    if (hidden.Value == "")
    {
        ClientScriptManager cs = Page.ClientScript;
        cs.RegisterStartupScript(this.GetType(), 'startupScript', 'drawImage();', true);
    }
}

More Info from SO

You can call the function from your button itself, without the postback. 

<asp:Button runat="server" ID='next' OnClientClick="return drawImage();" />

and in the script

function drawImage() {
    if(document.getElementById("hidden").value != undefined || document.getElementById("hidden").value != "")
    {
        context.drawImage(video, 0, 0, 320, 240);
        var imgBase = canvas.toDataURL('image/jpeg');
        document.getElementById("hidden").value = imgBase;
        return false;
    }
}
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